Formula to calculate rate over set months

Hi everyone! I have overthought this formula so much that I am completely stuck. This is the problem I'm trying to calculate. If you have any ideas, I would love to hear them.

Volunteers are recruited at .4 volunteers per month starting 1/1/2018.
They stay on with the company for 10 months and then roll off.
Recruitment ends 5/12/2019
The total project ends 3/10/2020 (and all volunteers should be gone at this time)
I need to determine how many volunteers will be with us monthly over the entire course of the project.

I will be graphing this data to present and I honestly have been working on it for hours and cannot manage to get the formula to be correct.

Thank you!!
I hope this file helps explain what I'm looking for. It seems like it should be a simple solution but apparently I have been staring at it for too long

Try

in H4

=IF(DATEDIF($C$3,H$2,"m")<12,G5+$G$3,IF(DATEDIF($C$3,H$2,"m")=12,G5-$G$3,IF(H$2<=$D$3,G5,G5-$G$3)))

G4 must be blank

Your post states recruits fall off after 10 months but it is 12 in your sample (if I read it correctly).